The present invention relates to an apparatus for controlling an internal combustion engine that has a variable valve actuation mechanism for varying the valve overlap state of intake and exhaust valves.
Internal combustion engines having a variable valve actuation mechanism have been proposed to improve the intake efficiency, decrease NOx, or exhaust emission, and improve the fuel consumption by reducing pumping loss caused by internal EGR. A variable valve actuation mechanism varies the actuation of intake and exhaust valves according to the running state of an engine. For example, variable valve timing mechanisms and variable valve lift mechanisms are widely used for on-vehicle internal combustion engines. A variable valve timing mechanism varies timings of intake and exhaust valves, or valve timing. A variable valve lift mechanism varies the valve lift of intake and exhaust valves.
In an engine having such a variable valve actuation mechanism, the amount of gas remaining in the cylinder, or the amount of internal EGR, changes according to operation of the mechanism, for example, according to changes in the valve overlap of the intake and exhaust valves. In consideration of changes in the amount of internal EGR according to operation of the variable valve actuation mechanism, settings of engine control parameters such as ignition timing are changed. For example, Japanese Laid-Open Patent Publication No. 9-209895 discloses an apparatus for controlling an internal combustion engine having such a variable valve actuation mechanism. The apparatus of the publication changes ignition timing in consideration of changes in the amount of internal EGR according to operation of the variable valve actuation mechanism. Valve overlap refers to a state where an intake valve and an exhaust valve are open at the same time.
The amount of internal EGR changes in a complicated manner according to the engine speed, the engine load, and the state of the variable valve actuation mechanism. However, the apparatus of the above publication does not clearly define the relationship between the state of the variable valve actuation mechanism and changes in the amount of internal EGR amount. Thus, the apparatus can hardly optimize the ignition timing in relation to changes in the internal EGR amount.
Since operation of the variable valve actuation mechanism affects the internal EGR amount, it is considered to be possible to control the internal EGR amount by the variable valve actuation mechanism. However, since the relationship between the state of the variable valve actuation mechanism and changes in the amount of internal EGR amount is not clearly defined, the internal EGR amount cannot be controlled with a high accuracy.